1. Field of the Invention
The present invention relates to a thin film transistor (TFT), a method of manufacturing the same, and a display device including the thin film transistor, and more particularly, to a thin film transistor having a coplanar structure with improves optical reliability and characteristics thereof, a method of manufacturing the thin film transistor, and a display device including the thin film transistor.
2. Discussion of Related Art
As interest in information display has recently increased and demands for portable electronic devices have increased, lightweight thin flat panel display devices have been widely studied and commercialized. From among flat panel display devices, particularly, liquid crystal display (LCD) devices and organic light-emitting display (OLED) devices have been particularly widely studied. LCD devices and organic light-emitting display devices use a thin film transistor as a switching device and/or a driving device.
Thin-film transistors using an oxide semiconductor are classified into thin film transistors having a coplanar structure and thin film transistors having an inverted-staggered structure according to positions of an active layer, a gate electrode, a source electrode, and a drain electrode. A parasitic capacitance between a gate electrode and an active layer in a thin film transistor having an inverted-staggered structure is very high. Because of such high parasitic capacitance, it is difficult to apply the thin film transistor having the inverted-staggered structure to a large-scaled display device. Therefore, coplanar type thin film transistors are employed in large-scaled display devices.
The active layer of a thin film transistor can be formed of amorphous silicon, poly-silicon or an oxide semiconductor. When a thin film transistor using poly-silicon is manufactured, a process of implanting ions to adjust a resistance of an active layer is additionally performed. Accordingly, an ion implantation process using an additional mask for defining an ion implantation region is disadvantageously added.
On the other hand, a thin film transistor using an oxide semiconductor has higher electron mobility than a thin film transistor using amorphous silicon, and exhibit lower leakage current than the thin film transistor using the amorphous silicon or the poly-silicon, and satisfies a high reliability test condition. Also, the thin film transistor using the oxide semiconductor advantageously has a uniform distribution of threshold voltages when compared to the thin film transistor using the poly silicon.
Despite such excellent electric and optical characteristics, the oxide semiconductor based TFT has a few drawbacks. For instance, the oxide semiconductor based TFT, especially the inverted-staggered type, exhibits high parasitic capacitance between the gate electrode and the active layer, which makes it difficult to be employed in the TFT in large display panels. In addition, the oxide semiconductor based TFTs may exhibit hysteresis (i.e., threshold voltage shift) when the oxide semiconductor material is exposed to incident light during the operation.
Thus, there remains a need for oxide semiconductor technologies that can enhance the performance and operation stability of TFTs.